Defaults + Type Classes

default_args_type_class.scala

/**
 * Attempting to factor this method (Which used to take a hard argument for Qry of BSONDocument
 * to use a type class, where the type class object SerializableBSONObject knows how to encode/decode the 
 * Document represented in Qry
 *
 * I was ***hoping*** That the Scala compiler would be smart enough to extract the type class type 
 * in the case of a Default Argument but it seems not.
 * The compiletime error is:
 * 
 * could not find implicit value for evidence parameter of type org.bson.SerializableBSONObject[Nothing]
 * 
 * A cookie and/or mug of tasty beer for anyone who has clever ideas that don't involve 
 * java style overloaded methods
 */
def findAndRemove[Qry :  SerializableBSONObject](db: String)(collection: String)(query: Qry = Document.empty) = findAndModify(db)(collection)(query=query, remove=true)_

I am not seeing the org.bson.SerializableBSONObject[Nothing].

trait Qry {}
class Document extends Qry {
}
object Document {
  def empty = new Document
}
trait SerializableBSONObject[A] {}
object SerializableBSONObject {
  implicit val serialize: SerializableBSONObject[Document] = new SerializableBSONObject[Document] {}
}
object Foo {
  def findAndRemove[Qry: SerializableBSONObject](db: String)(query: Qry = Document.empty) = "something"
}
object Main {
  def main(args: Array[String]) {
    println (Foo.findAndRemove("")() )
  }
}

My guess is it's the usage of findAndModify that's burning you here.

Ya, I found the problem. findAndModify has a default value for an Update parameter of Option[T] = None I changed it to Option[T] = Option[Document](null) That fixes it ;)

…

On Thu, Jun 9, 2011 at 12:21 PM, jsuereth < ***@***.***>wrote: My guess is it's the usage of findAndModify that's burning you here. ## Reply to this email directly or view it on GitHub: https://gist.github.com/1016955